Cycles Gradient Texture node: Why "Linear" is not linear on a "default" plane?
I think that the gradient is linear in its values because at equal increments of the thereshold correspond the same amount of image masking:



As stated by the Blender reference manual's page about Color Management,  Default option does no extra conversion..


  ..besides the conversion for the display device


So a converision still occurs.

By switching to Raw you'll get a result almost identical to the one expected. Again from the same manual page:


  Raw gives the image without any color space conversion




On the right you can see a linear gradient image for comparison.



Object coordinates are calculated from object's origin.

.

If you want a gradient that starts from the bottom of the plane, you should move the origin of the object's texture coordinates (so the Object's  origin) at the bottom of the plane too (the default plane's origin is instead located in its center).